The issue discussed in this old one:
and this less old one:
just had me tearing hair out for a cool minute.
To summarize: I accidentally hit the little "< Collapse Menu" button, which shrinks the sidebar to icons-only view. So I intentionally clicked it a second time to make it expand. But on the next page I clicked away to, it went ahead and collapsed itself, then from that point on it would refuse to stay expanded.
Logged into the site in another browser--> same behavior. Drat!
Created a new admin user just to check--> when logged into that user, the sidebar behaved as expected. I clicked the button to expand/collapse it several times, navigate to different pages, all worked as it supposed to.
Logged back into the other (main) admin, and issue was still there. I tried monkeying with that user a bit, changing the password, changing it's role... nothing help, it's like that user was cursed to to have a collapsed menu forever.
So I took the path of least resistance: logged into the new admin, deleted the affected user (attributing all the posts to this new user), then recreated it, using the same email, password and username.
Now when I log into this recreated user, all is well. I could even click the little button and it would do what it's supposed to. All's well; it's just 20 minutes or so that I can't get back. Oh well.
Just thought I'd share my resolution in case someone else is going hairless out there fighting with their sidebar menu that won't stay open.